This invention relates to a method for determining the relative temporal position of electromagnetic pulses and to a determination device for carrying out such method.
From the prior art, it is known to determine the relative temporal position (phase) of two electromagnetic pulses (or two pulse trains), in order to be able to adjust or control the temporal position of the pulses (e.g. two light pulses). For example, measurements will be made by varying the relative temporal position of the pulses, e.g. in pump-and-probe measurements. Here, a first pulse (pump pulse) excites an effect (i.e. it causes an electromagnetic pulse) and a second, variably delayed pulse (probe pulse) samples the time behavior of the electromagnetic pulse (i.e. of the effect) caused by the first pulse.
For example, terahertz pulse systems also operate according to the pump-probe principle: A first pulse excites a photoconducting transmitting antenna to emit terahertz radiation and a second pulse samples the signal received from a second photoconducting detector antenna with variable delay. Other pump probe measurements proceed from a pulse source (e.g. a pulse laser) whose pulsed radiation is split up into two paths, wherein in one of the two paths a (e.g mechanical) delay line is arranged. Instead of a (relatively slow) delay mechanism, other systems employ an arrangement for varying the repetition frequency of the pulse source: When a first pulse source with a repetition frequency f1 generates the pump pulses and a second pulse source with a repetition frequency f2 generates the probe pulses, the relative temporal position of the pulses remains constant when the repetition frequencies f1 and f2 are equal. When one of the frequencies becomes smaller, the corresponding pulses overtake the other pulses; when the frequency becomes larger, the pulses fall back again. However, this method requires a synchronization of the two pulse sequences generated by the two pulse sources.
One possibility of synchronizing the two pulse sequences consists in the direct detection of the pulses with detectors which cannot resolve the pulses temporally, but can follow the repetition rate. With downstream electronic microwave components and techniques the phase and/or frequency deviations then are detected and control signals are generated (J. Posthumus, “Terahertz with Electronic Delay”, Optik-Photonik, Wiley, 2007, pp. 29-31).
It is also known to irradiate the pulse sequences into a non-linear crystal as “time correlator”, wherein the sum frequency (or double the frequency) of the electromagnetic light wave only is obtained when the pulse sequences simultaneously and synchronously pass through the non-linear crystal. The power of the radiation generated in the non-linear crystal therefore indicates the time correlation of the two pulse sequences (J. Kim et al., Optics Letters, Vol. 32, 9, pp. 1044-1046, 2007).